Broken coverage on 3.8-dev

[njsmith]

So apparently codecov's weird behavior for the last few weeks is caused by an interaction between coverage.py and this change in 3.8-dev: python/cpython#8774

Another example: marrink-lab/vermouth-martinize#132

The coverage devs are aware of it: nedbat/coveragepy#707 (comment)

We should maybe disable coverage tracking on 3.8-dev builds until this gets fixed.